1. Field of the Invention
The present invention relates to dual-core and multicore optical fibers, and in particular, to the simultaneous accessibility of multiple cores in the same fiber.
2. Description of the Related Art
A dual-core fiber, such as that in the prior art, is depicted in FIGS. 1 and 2. Two cores 12 and 14 within a fiber 16 support respective propagating optical beams 18 and 20. The term "optical beam" is used broadly herein to include electromagnetic radiation propagating in air or space as well as electromagnetic modes propagating in fiber cores. The cores 12 and 14 are surrounded by a cladding 17. The cores 12 and 14 are separated by a center-to-center spacing of d and have respective diameters a.sub.1 and a.sub.2. The index of refraction of the cladding 17 is lower than the indices of refraction of the cores 12 and 14 so that optical energy is guided within the cores through total internal reflection. If the cores 12 and 14 are made from the same fiber preform, and the fiber drawing is done such that the cores are identical, the fiber 16 is known as a twin-core fiber. Upon reaching the fiber's flat output face 22, optical energy diverges from each of the cores 12 and 14. The maximum angle at which the optical radiation from a core can exit the flat face 22 is related to the core's numerical aperture (NA): EQU NA=.sqroot.n.sub.co.sup.2 -n.sub.cl.sup.2 (1)
in which n.sub.co and n.sub.cl are the indices of refraction of the core and cladding, respectively.
The range of angles .theta. about an axis perpendicular to the output face 22 at which refracted rays may exit can be determined with Snell's law to be: ##EQU1## in which n.sub.ext is the index of refraction of the medium, e.g., air, into which the beams 18 and 20 propagate. This range of angles .theta. is illustrated in FIG. 1 with marginal rays 24 and 26 for the core 12 and marginal rays 28 and 30 for the core 14. The beams 18 and 20 spatially overlap in the far field, where an interference fringe pattern may be formed if the two beams are mutually coherent at the output face 22. Thus, the powers in the cores 12 and 14 cannot be individually monitored, unless the fiber 16 is physically split between the cores 12 and 14 so that the cores are separated from each other as illustrated in FIG. 2. (See, for example, G. Schiffner, et al., "Double-Core Single-Mode Optical Fiber as Directional Coupler," Applied Physics, Vol. 23, pp. 41-45, 1980.) Likewise, the input end of the fiber 16 must generally be split to allow separate optical inputs into the cores 12 and 14. To fabricate the fiber 16 depicted in FIG. 2, holes may be drilled in the fiber preform between the two preform cores 12 and 14, at some given spacing. When the fiber 16 is drawn, these holes extend to form the split in the `Y` depicted in FIG. 2. Physical limitations on the possible spacing of drilled holes in the preform determine the minimum length of dual-core fiber 16 which may be connected by two such split and drawn regions. These lengths cannot be controlled to the accuracy needed in some applications, i.e., couplers. In addition, typically, these lengths are fairly long.
Dual-core fibers are of interest because of their use as couplers, wavelength division multiplexers, and sensors. (See, for example, G. Schiffner, et al., supra, 1980; and G. Meltz, et al., "Cross-talk fiber-optic temperature sensor," Applied Optics, Vol. 22, pp. 464-477, 1983.) The aforementioned problem with coupling into and out of such fibers has severely limited their application, however. Unless both cores can be simultaneously accessed at both the input end and the output end, a dual-core fiber cannot be used as a practical 2.times.2 device. Thus, applications to this point have not been able to exploit the full potential of dual-core or multiple-core fibers.
Dual-core fibers are currently used in interferometric measurements in which power is launched into one core of the fiber or split evenly between the cores. (See, for example, J. W. Arkwright, et al., "Nonlinear Phase Changes at 1310 nm and 1545 nm Observed Far From Resonance in Diode Pumped Ytterbium Doped Fiber," IEEE Photonics Technology Letters, Vol. 8, pp. 408-410, 1996; J. Arkwright, et al., "Enhanced Switching Speeds observed at 980 nm in neodymium doped twin-core fibre, using simulated downpumping at 1060 nm," Fiber Laser Sources and Amplifiers, SPIE Vol. 2073, pp. 158-165, 1993; and P. L. Chu, et al., "Optical switching in twin-core erbium-doped fibers," Optics Letters Vol. 17, pp. 255-257, 1997.) In these switching devices, interference fringes and fringe shifts are used to determine when a relative phase shift has occurred between the fields propagating within the two cores. (See, for example, J. W. Arkwright, et al., 1996, supra, and J. W. Arkwright, et al., 1993, supra). A slit is used to isolate a single output fringe, or a pinhole to isolate a fraction of an output fringe. Thus, a significant portion of the output power is discarded, leading to a lower signal-to-noise ratio in such a measurement. Even coupling measurements for determining the fraction of power coupled from one core of a twin-core fiber to the other core over a fixed distance rely on such an interferometric measurement rather than a direct power measurement. (See, for example, G. D. Peng, et al., "Accurate elasto-optic probe method for measurement of coupling length in twin-core optical fiber," Applied Optics, Vol. 33, pp. 1004-1010, 1994.)
Power in one core can be accessed by butt coupling a separate single-core fiber to one of the cores in the dual-core fiber (see, for example, P. L. Chu, et al., supra, 1997), but this requires a cumbersome alignment procedure. There is not enough physical space to butt-couple two single-core fibers side by side onto the same end of a dual-core fiber, so that it is also not possible to couple this way from both cores. Thus, the development of dual-core fibers as four-port devices has been hindered.